The present invention relates to a process for the production and recovery of crystalline sodium sesquicarbonate and of crystalline sodium carbonate monohydrate from aqueous liquors containing sodium carbonate and sodium bicarbonate and, particularly, from aqueous liquors obtained from the solution mining of subterranean trona ore deposits.
Sodium carbonate, also called soda ash, is an important, high volume chemical produced in the United States and used in the manufacture of glass, chemicals, soaps and detergents and aluminum, as well as in textile processing, petroleum refining and water treatment, among many other uses.
In the United States, almost all sodium carbonate is obtained from subterranean deposits of naturally occurring trona ore. The largest known trona ore deposits in the United States are located in Green River, Wyo. and are typically about 800 to 3000 feet below ground level; these trona ore deposits are actively mined by several companies.
Trona ore consists primarily (80-95 percent) of sodium sesquicarbonate (Na2CO3.NaHCO3.2H20), and lesser amounts of sodium chloride (NaCl), sodium sulfate (Na2SO4), organic matter, and insolubles such as clay and shales. A typical analysis of crude trona ore being mined at Green River, Wyo. is as follows:
ConstituentsWeight Percent (wt %)sodium sesquicarbonate90sodium chloride (NaCl)0.1sodium sulfate (Na2SO4)0.02organic matter0.3insolubles (clay and shales)9.6
Trona ore may be recovered from subterranean deposits, for further processing into soda ash, by mechanical mining techniques or by any of several various solution mining methods. The Green River trona ore deposits are presently being commercially mined both by mechanical mining and by solution mining processes.
Mechanical mining, also called dry mining, is carried out underground in the ore beds by mining crews and include room-and-pillar and long wall methods. Mechanical mining methods are relatively costly and leave unrecovered a significant fraction of the trona ore in the beds being mined.
Solution mining processes represent an economical alternative to mechanical mining, for recovery of the same sodium carbonate values from subterranean trona ore deposits. Solution mining involves injecting water or other aqueous-based mining solution, via a drilled well hole, into a deposit of trona ore; allowing the mining solution to dissolve soluble ore; pumping the resulting mining solution (minewater) via a drilled well hole to the surface; and processing the minewater to recover the dissolved ore values from the solution in the form of sodium carbonate or other related sodium based chemicals. Solution mining methods are also useful for recovering alkali values from depleted ore deposits that have previously been mechanically mined.
As its chemical composition indicates, trona ore requires processing in order to recover sodium carbonate. Two conventional surface process methods for recovery of sodium carbonate (soda ash) are the “sesquicarbonate” process and the “monohydrate” process, named after their respective crystallization products.
The sesquicarbonate process involves dissolution of mechanically-mined and crushed trona ore in a recycled hot aqueous liquor containing a molar excess of dissolved carbonate over bicarbonate, in order to effect congruent dissolution of the sodium sesquicarbonate in the trona ore; clarifying and filtering the ore-fortified solution to remove insoluble matter present in the ore; introducing the filtered aqueous liquor into a series of vacuum crystallizers to crystallize sodium sesquicarbonate by evaporation of water and cooling; withdrawing crystallizer slurry and recovering crystallized sodium sesquicarbonate by centrifugation from the crystallizer slurry; calcining the recovered sodium sesquicarbonate crystals at elevated temperature to convert the sesquicarbonate to a soda ash product having light bulk density; and recycling the crystallizer mother liquor to dissolve additional crude trona, as first described.
The “monohydrate” process was developed in response to the need for a more dense soda ash than that produced by the sesquicarbonate process. Most of the soda ash recovered from the Green River trona ore deposits is currently produced from mechanically mined trona ore via the monohydrate process or variants thereof.
The monohydrate process involves crushing and screening the bulk trona ore which, as noted above, contains sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O) as well as impurities such as silicates and organic matter. The crushed and screened trona ore is then calcined, i.e., heated, at elevated temperatures greater than about 150° C. to convert the ore's sodium bicarbonate content to a crude sodium carbonate.
The crude sodium carbonate is dissolved in a recycled aqueous liquor which is clarified and filtered to remove the insoluble solids. The aqueous liquor is sometimes carbon treated to remove dissolved organic matter which may cause foaming and color problems in the final product, and is again filtered to remove entrained carbon. The aqueous liquor, fortified with dissolved sodium carbonate from the calcined ore, is then fed to a high temperature evaporative crystallization circuit generally having one or more effects (evaporators), to evaporate water and crystallize the desired product, sodium carbonate monohydrate, as the stable crystal phase. The resulting crystal slurry is withdrawn from the crystallizer, centrifuged, and the separated monohydrate crystals are sent to dryers to remove the water of hydration and to produce a dense soda ash product.
The crystallizer mother liquor remaining after separation and recovery of the monohydrate is recycled as an aqueous liquor stream, as first described, to dissolve additional calcined ore. A portion of the recycled mother liquor stream is purged to prevent the buildup of soluble impurities in the recycling aqueous liquor.
Numerous solution mining techniques are described in the prior art. In many of these prior art solution mining processes, a primary objective was to maximize solubilization of the trona ore in the mining solvent or to otherwise provide a concentrated aqueous effluent solution, or brine, for further processing to recover soda ash. Examples of these prior art approaches include the use of sodium hydroxide-containing mining solutions or fortification of an aqueous effluent with added sodium carbonate values. The resulting concentrated solutions may then be processed into soda ash, for example, by using conventional surface processing techniques such as the sesquicarbonate process or the monohydrate process.
Other solution mining methods, that do not rely on producing soda ash via the sesquicarbonate or monohydrate processes, are also described in the prior art for processing of solution mining effluent liquors. These prior art methods typically involve complex procedures, involving multiple steps in which various forms of sodium carbonate are crystallized either as alternatives to sodium carbonate monohydrate or sodium sesquicarbonate, or as intermediates used to make these latter products. These additional intermediate steps not only add to the complexity of the overall process but also add significant economic costs and inefficiencies to these soda ash recovery techniques.
The solution mining method of choice to recover soda ash from trona ore deposits or from other Na2CO3-containing ore deposits must be reconciled with one marketplace factor.
The type of soda ash most in demand in the marketplace is dense soda ash made from sodium carbonate monohydrate. Demand also exists for light density soda ash made from calcined sodium sesquicarbonate, and for sodium sesquicarbonate per se. As a result, other sodium carbonate species made in prior art processes, e.g., sodium carbonate decahydrate and true anhydrous sodium carbonate, do not represent significant factors in the marketplace. Prior art solution mining processes which make such species are typically inefficient, since these sodium carbonate-based entities become intermediates used to make the desired monohydrate soda ash product.
Prior art solution mining processes that illustrate these drawbacks include the following, by way of example.
In U.S. Pat No. 5,262,134, Frint et al. disclose a process for producing soda ash from a solution mining effluent brine containing sodium carbonate and sodium bicarbonate by first crystallizing and recovering sodium sesquicarbonate as a first product. Next, sodium carbonate decahydrate is crystallized at low temperature and recovered from the sesquicarbonate crystallizer mother liquor that remains after separation of the sesquicarbonate product. The recovered sodium carbonate decahydrate is heated and redissolved in solution, and then sodium carbonate monohydrate is crystallized, recovered and dried to make dense soda ash as a second product.
In U.S. Pat No. 5,283,054, Copenhafer et al. disclose a method for producing soda ash from a brine produced by solution mining, in which sodium carbonate decahydrate is again produced as an intermediate crystalline product. Instead of an initial sesquicarbonate crystallization step, the solution mining effluent brine is first heated to evaporate water and decompose a portion of sodium bicarbonate. The concentrated and CO2-stripped stream is then treated with caustic soda to neutralize sodium bicarbonate still remaining in solution. Sodium carbonate decahydrate is crystallized from the NaHCO3-depleted solution at low temperature, recovered and then redissolved by heating. Sodium carbonate monohydrate is crystallized from the decahydrate-derived solution, recovered and dried to make a dense soda ash product.
A hybrid solution mining process is disclosed in U.S. Pat. Nos. 6,207,123 and 6,576,209 of Tanaka et al., in which soda ash is obtained from sodium carbonate monohydrate that is ultimately crystallized from a fortified aqueous stream. The aqueous stream may be obtained from solution mining of trona ore and is fortified with crude sodium carbonate calcinate obtained from calcined dry mined ore. Crude sodium sesquicarbonate is also crystallized as an intermediate; this intermediate is preferably calcined and reincorporated into the fortified aqueous liquor to provide additional crude sodium carbonate values. The sodium carbonate values in the recovered soda ash product (dehydrated sodium carbonate monohydrate) are sourced only in part from the solution mining liquor, with calcined dry mined trona ore being added to the solution mining effluent liquor—the latter calcinate provides approximately half of the recovered soda ash values in the example.